The selection of cold-adapted mutants of selected human rotaviruses of major epidemiologic importance was reported last year. These mutants included strain D (VP4:1A; VP7:1), DS-1 (VP4:1B; VP7:2), Wa x DS-1 (VP4:1A; VP7:2), and Wa x P (VP4:1A; VP7:3). Selected mutant vaccines are currently under clinical evaluation. In an attempt to provide antigenic coverage for each of the four epidemiologically important VP7 serotypes (G1, G2, G3, and G4), we passaged a Wa x ST3 (VP4:1A ; VP7:4) reassortant serially in primary African green monkey kidney cells at progressively lower suboptimal temperature (30 degrees C, 28 degrees C, and 26 degrees C). Triple plaque purification of each mutant selected after tenth serial passages is in progress at the temperature of that passage series.